1. Field Of The Invention
The invention pertains to controlling the strike positions of several electron beams on a melting bath, whereby electromagnetic radiation is concentrated at the strike points.
2. Prior Art
When heating, melting or evaporating metallic or other materials, energy-rich electron beams are often arranged to strike the material to be heated, melted or evaporated. In general, it is necessary in this case to position the electron beam exactly in a predetermined manner. Thus, for example, the evaporation energy sources of the electron beam can be moved over the surface of the material to be evaporated such that as even a surface temperature as possible is reached on a specific surface, and therefore, a constant evaporation rate is maintained. In electron beam melting units, it is important that the electron beam does not strike beside the material to be melted. When electron beam welding guns are used, the electron beam must be guided exactly along the separation line of the parts to be welded.
The electron beam is guided to the desired position in the units mentioned by electric and/or magnetic fields. The position of the strike point of the electron beam is determined by the appropriate dimensioning and adjusting of an electric or magnetic deflecting system as well as by the application of appropriately regulated electric voltage or current to the deflecting system.
During drip melting by means of electron beams, especially where multiple remelting steps are involved, the remelting process is carried out by means of central-vertically guided electrodes and by using several electron beam guns. Modern remelting furnaces are generally limited to two electron beam guns, which are mounted opposite the consumable electrode. This limitation is possible because two semicircular beam distributions guarantee that the consumable electrode and the crucible bath are symmetrically stressed.
A device for the recognition of the strike point of an electron beam is already known, in which the X-rays emitted by a striking electron beam are applied to a lateral diode and cause this diode to produce streams which are directed the X and Y positions of the strike point of the electron beam (EP-A-0,184,680). This device has the advantage of an exact determination of location, but it is relatively expensive and has sensitive components, e.g., thin sheets used as separators between the atmosphere and a vacuum, which represent a certain danger for the safety of the melting process.
Moreover, a vacuum evaporation unit for the discontinuous evaporation of turbine vanes is known, which has a vacuum chamber, in the upper chamber wall of which two electron beam guns are mounted, which are provided with electrical energy through a control unit (DE-A-2,812,285).
As has been determined with such two-gun units, it can happen that in the process of increasing output, the position of beam distribution in the crucible, which is lined up to two semicircular illuminations of the melting bath, moves without a change occurring in the radial deflecting system. The old position of beam distribution in the crucible can be re-established through a manual correction of the radial deflection. This effect, which depends on the material to be melted, is apparently caused by the asymmetrical influence of the ions from the consumable electrode and the melting bath on the beams in the proximity of the melt. The effects occurs, in particular, with initial melts of retracting metals which have a high gas content.